What is the "aft-loaded" wing found on the
Boeing 767 and
Boeing 777? How does it differ from and how is it more
efficent than a conventional wing?- question from Steve

The term "aft loaded" refers to where the lift force acts on a wing or airfoil. If we were to look at a side
profile of a wing (called an airfoil), we could plot the distribution of the pressure acting over the shape, a
quantity directly related to the lift. On most traditional airfoil shapes, such as the early
NACA series, this distribution tends to be concentrated towards the forward
portion of the airfoil, as exemplified below. To understand this figure, you must realize that the distance
between the two lines represents the amount of lift acting at that point along the airfoil section.

Pressure distribution over a conventional airfoil

While these sorts of airfoil shapes worked well on planes up through the start of World War II, they were not nearly
as efficient on newer aircraft that approached the speed of sound. As discussed in a previous question on
high speed flight, shock waves begin to form on airfoils as they enter the transonic speed regime starting at about
Mach 0.7. These shock waves result in large increases in drag and may interfere with control surfaces thereby
drastically degrading a plane's performance. A new kind of airfoil was needed to improve performance at these high
speeds, a shape called the supercritical airfoil. While we will not repeat the
aerodynamic advantages of these gemoetries here, what is important to us in this discussion is how the pressure
is distributed over the surface of a supercritical airfoil. As we can see in the example below, the pressure is
distributed much more evenly with a large portion of the lift concentrated towards the aft end of the shape.

Pressure distribution over a supercritical airfoil

We therefore say that the supercritical airfoil is "aft-loaded" because the lift force is more significantly
"loaded" onto the aft portion of the shape. When Boeing refers to the 767 and 777 as having aft-loaded wings,
what they are implying is that both aircraft use supercritical airfoils. This seems reasonable since the cruise
speeds of both aircraft are about Mach 0.8, within the transonic regime, and these are exactly the sort of aircraft
we would expect these airfoils to be applied to. And as we have already mentioned, these aft-loaded wings are more
"efficient" than a conventional wing because they reduce or delay the adverse effects of shock waves in transonic
flight.
- answer by Jeff Scott, 25 November 2001